Diesel engines operate by compression ignition. They have compression ratios in the range of 14:1 to 17:1 or higher and for that reason obtain more useful work from a given amount of fuel compared to an Otto cycle engine. Historically, diesel engines have been operated on a petroleum-derived liquid hydrocarbon fuel boiling in the range of about 300.degree.-750.degree. F. Recently, because of dwindling petroleum reserves, alcohol and alcoholhydrocarbon blends have been studies for use as diesel fuel.
One major factor in diesel fuel quality is cetane number. Cetane number is related to ignition delay after the fuel is injected into the combustion chamber. If ignition delays too long, the amount of fuel in the chamber increases and upon ignition results in a rough running engine and increased smoke. A short ignition delay results in smooth engine operation and decreases smoke. Commercial petroleum diesel fuels generally have a cetane number of about 40-55. Alcohols have a much lower cetane value and require the addition of a cetane improver for successful engine operation.
Through the years, many types of additives have been used to raise the cetane number of diesel fuel. These include peroxides, nitrites, nitrates, nitrosocarbamates, and the like. Alkyl nitrates such as amyl nitrate, hexylnitrate and mixed octyl nitrates have been used commercially with good results.